The present invention relates to an aqueous emulsion that comprises a polyvinyl alcohol having at least 1.9 mol % of 1,2-glycol bonds (this will be hereinafter referred to as the first aspect of the invention), and to a dispersant for suspension polymerization that comprises the polyvinyl alcohol (this will be hereinafter referred to as the second aspect of the invention).
According to the invention, the polyvinyl alcohol having at least 1.9 mol % of 1,2-glycol bonds is used as a dispersant for emulsion polymerization or a dispersant for suspension polymerization, and this ensures good polymerization stability even when its amount used is reduced; the aqueous emulsion comprising the polyvinyl alcohol is resistant to water and the viscosity of the aqueous emulsion depends little on ambient temperatures; the vinylic polymers obtained through suspension polymerization in the presence of the polyvinyl alcohol are yellowed little; and the waste water in the suspension polymerization pollutes little the environment.
 less than 1 greater than  Background Art of the First Aspect of the Invention, Aqueous Emulsion
Heretofore, polyvinyl alcohol (hereinafter referred to as PVA) is widely used as protective colloid in emulsion polymerization of an ethylenic unsaturated monomer, especially a vinyl ester monomer such as typically vinyl acetate. Aqueous emulsions of vinyl ester polymers obtained through emulsion polymerization with PVA serving as protective colloid have many applications in various fields of adhesives for paper products, wood products, plastics, etc., binders for dip-coated paper, non-woven fabrics, etc., as well as admixtures, jointing agents, coating compositions, paper processing agents, fiber processing agents, etc.
Controlling the degree of hydrolysis of PVA therein makes it possible to change the physical properties of such aqueous emulsions, some of which will have low viscosity just like Newtonian fluid, and will be relatively highly resistant to water, while some others will have high viscosity relatively independently of ambient temperatures. Depending on such their properties, aqueous emulsions containing PVA have many applications in various fields.
However, some aqueous emulsions containing PVA have drawbacks in that they are not resistant to water and that their viscosity greatly varies depending on ambient temperatures.
PVA serving as a dispersant for emulsion polymerization is generally grouped into xe2x80x9ccompletely hydrolyzed PVAxe2x80x9d having a degree of hydrolysis of 98% or so and xe2x80x9cpartially hydrolyzed PVAxe2x80x9d having a degree of hydrolysis of 88 mol % or sol Aqueous emulsions containing the former PVA could have relatively good water resistance but are problematic in that their viscosity greatly varies depending on ambient temperatures. On the other hand, the viscosity of aqueous emulsions containing the later PVA does not so much depend on ambient temperatures but its temperature dependency is not still negligible, and, in addition, the aqueous emulsion is are poor in their water resistance. To solve the problems with these aqueous emulsions containing PVA, using the two types of PVA as combined in aqueous emulsions, or using PVA having an intermediate degree of hydrolysis therein is tried. However, no one has heretofore succeeded in realizing aqueous, PVA-containing emulsions that satisfy the two requirements of good water resistance and temperature independence for their viscosity. In that situation, PVA with ethylene units therein has been proposed for use in aqueous emulsions (Japanese Patent Laid-Open No. 81666/1996). Containing it, aqueous emulsions having improved water resistance and low-temperature storage stability. However, their water resistance is not satisfactory as yet and their temperature independence is not also satisfactory (see Comparative Example 7 given hereinafter). Also known is using a mercapto-terminated PVA as a dispersant for emulsion polymerization, as in Comparative Example 6 given hereinafter (Japanese Patent Laid-Open No. 24481/1991). As in Comparative Example 6, however, this is not satisfactory in point of its water resistance, and its temperature dependence of the viscosity is not satisfactorily lowered.
Given that situation, the first aspect of the invention is to provide an aqueous emulsion having the advantages of improved water resistance and lowered temperature dependence of the viscosity, and to provide a method for producing such an aqueous emulsion having the advantage of good polymerization stability, especially that still having the advantage of good polymerization stability even when the amount of the dispersant added is reduced.
 less than 2 greater than  Background Art of the First Aspect of the Invention, Dispersant for Suspension Polymerization
For producing vinylic polymers such as polyvinyl chloride resins and the like on an industrial scale, for example, widely employed is a method of suspension polymerization that comprises dispersing a vinylic compound such as vinyl chloride or the like in an aqueous medium in the presence of a dispersant followed by polymerizing it in the presence of an oil-soluble initiator. In general, the factors that govern the quality of vinylic polymers produced through such suspension polymerization include the polymerization conversion, the ratio of monomer to water, the polymerization temperature, the type and the amount of the initiator, the type of the polymerization reactor, the stirring speed, and the type of the dispersant. Above all, the type of the dispersant has the most significant influence on the polymer quality.
The necessary properties of the dispersant for suspension polymerization of vinylic compounds are that  less than 1 greater than  it is effective for narrowing as much as possible the particle size distribution of the vinylic polymer particles obtained,  less than 2 greater than  it is effective for making the polymer particles porous in order that the polymer particles can rapidly absorb a plasticizer so as to be readily processed, that the monomer such as vinyl chloride or the like remaining in the polymer particles can be readily removed, and that the shaped articles of the polymer particles are prevented from having fish eyes and other defects therein,  less than 3 greater than  it is effective for making the polymer particles have a large bulk density,  less than 4 greater than  it ensures stable polymerization even when its amount added is small, and the waste water in the process of polymerization pollutes little the environment, and  less than 5 greater than  it does not have any negative influence on the heat resistance of the vinylic polymers obtained.
Vinylic compounds such as vinyl chloride are generally polymerized in a mode of batch-system suspension polymerization. One typical process of batch-system suspension polymerization of vinylic compounds comprises feeding an aqueous medium, a dispersant, a polymerization initiator and a vinylic compound into a polymerization reactor, then optionally adding necessary additives thereto, and heating the reactor to polymerize the monomer therein. For improving the productivity in the process, the recent tendency in the art is toward shortening the time to be taken in one batch polymerization. For this, for example, a reflux condenser or the like is installed in the system of suspension polymerization of vinylic compounds to thereby increase the efficiency of removing polymerization heat from the system; or an aqueous medium having been previously heated is charged into the system to shorten the heating time in the system (hot-charge process). However, conventional dispersants for suspension polymerization of vinylic compounds foam greatly in polymerization reactors to reduce the effective reactor capacity and to lower the polymer productivity. In particular, when such a conventional dispersant is used in a polymerization reactor equipped with a reflux condenser, it interferes with temperature control in the reactor; or when it is used in the hot-charge process, the vinylic polymer particles produced could not be porous. These are fatal defects of conventional dispersants. On the other hand, if a defoaming agent is added to the polymerization system to prevent it from foaming, it presents another problem in that the heat stability of the vinylic polymer particles produced is lowered.
For dispersants for suspension polymerization of vinylic compounds, heretofore used are cellulose derivatives such as methyl cellulose and carboxymethyl cellulose, and partially hydrolyzed PVA, either singly or as combined. In Poval (published by the Polymer Publishing in 1984xe2x80x94reference (a)), described is PVA having a degree of polymerization of 2000 and a degree of hydrolysis of 80 mol %, and this serves as a dispersant for suspension polymerization of vinyl chloride. In WO 91/15518 (reference (b)), described is a dispersant for suspension polymerization of vinylic compounds, which comprises PVA having an amino group, an ammonium group, a carboxyl group or a sulfonic acid group at the terminal and having a degree of polymerization of at least 100 and a degree of hydrolysis of from 50 to 90 mol %. In Japanese Patent Laid-Open No. 25307/1997 (reference (c)), described is a dispersant for suspension polymerization of vinylic compounds, which comprises PVA having a hydroxyalkyl group. In Japanese Patent Laid-Open No. 283313/1996 (reference (d)), described is a dispersant for suspension polymerization of vinylic compounds, which is specifically defined in point of the UV absorbance of its aqueous solution and the block character of the acetic acid group remaining therein. In Japanese Patent Laid-Open No. 115587/1976 (reference (e)), described is a method of producing polyvinyl chloride resins in the presence of a partially hydrolyzed PVA. The partially hydrolyzed PVA used therein has a degree of hydrolysis of from 60 to 90 mol %, and the degree of methanol release from it at temperature of 55xc2x0 C. falls between 10 and 60%. In Japanese Patent Laid-Open No. 110797/1977 (reference (f)), described is a secondary dispersant for suspension polymerization of vinyl chloride, which comprises a polyvinyl acetate having a degree of hydrolysis of from 30 to 60 mol %. However, the dispersants described in reference (a) and reference (b) do not satisfy the requirements  less than 1 greater than  to  less than 4 greater than  mentioned above, and, in addition, they are further defective in that they foam greatly in polymerization systems. The dispersants described in reference (c) and reference (d) are good is some degree, as they do not foam so much in polymerization systems and their plasticizer absorbability is good. However, they do not satisfy the requirements  less than 1 greater than  and  less than 3 greater than  to  less than 5 greater than  mentioned above. The dispersant used in the method described in reference (e) has relatively well balanced properties in point of the requirements  less than 1 greater than  to  less than 3 greater than , but does not satisfy the requirements  less than 4 greater than  and  less than 5 greater than . In addition, it is defective in that it greatly foams in polymerization systems. The secondary dispersant described in reference (f) cannot stabilize the system of suspension polymerization of vinylic compounds, when used alone. In addition when it is combined with a conventional partially hydrolyzed PVA, it presents a serious problem in that the polymerization system in the reactor greatly foams.
The object of the second aspect of the invention is to provide a dispersant for suspension polymerization of vinylic compounds, of which the advantages are that it enables stable polymerization of vinylic compounds, that its amount to be used can be reduced and therefore the vinylic polymers obtained through suspension polymerization in the presence of it are yellowed little and the waste water in the polymerization system does not so much pollute the environment, that it does not foam the polymerization system in a reactor, and that it satisfies all the requirements mentioned above.
 less than 1 greater than  First described is the first aspect of the invention, aqueous emulsion.
The object of the first aspect of the invention can be attained by providing an aqueous emulsion which comprises, as the dispersoid, a polymer that contains vinyl ester monomer units, and, as the dispersant, a polyvinyl alcohol that contains at least 1.9 mol % of 1,2-glycol bonds, and of which the viscosity profile is such that the ratio of T0xc2x0 C./T30xc2x0 C. is at most 5 and the ratio of T60xc2x0 C./T30xc2x0 C. is at most 1.5 with T60xc2x0 C. indicating the viscosity of the aqueous emulsion at 60xc2x0 C., T30xc2x0 C. indicating the viscosity thereof at 30xc2x0 C. and T0xc2x0 C. indicating the viscosity thereof at 0xc2x0 C.
The polyvinyl alcohol having at least 1.9 mol % of 1,2-glycol bonds and serving as the dispersant in the aqueous emulsion of the invention is not specifically defined for its production, and may be produced in any known method. For example, it may be produced by copolymerizing a vinylene carbonate with a vinyl ester to have the 1,2-glycol bond content as above; or by polymerizing a vinyl ester at a temperature higher than that for ordinary vinyl ester polymerization, for example, at a temperature falling between 75 and 200xc2x0 C., under pressure. In the latter method, the polymerization temperature preferably falls between 95 and 190xc2x0 C., more preferably between 100 and 180xc2x0 C. Regarding the pressure for polymerization, it is important that the pressure is so controlled that the temperature of the polymerization system does not exceed the boiling point thereof. For this, preferably, the polymerization pressure is at least 0.2 MPa, more preferably at least 0.3 MPa. Its uppermost limit is preferably at most 5 MPa, more preferably at most 3 MPa. Under the condition, the monomer may be polymerized in the presence of a radical polymerization initiator in any mode of bulk polymerization, solution polymerization, suspension polymerization or emulsion polymerization, for which, however, preferred is solution polymerization, and especially preferred is solution polymerization with a solvent of methanol. The vinyl ester polymer thus produced is hydrolyzed in an ordinary manner to obtain a polyvinyl alcohol for use herein. The 1,2-glycol bond content of the polyvinyl alcohol must be at least 1.9 mol %. More preferably, it is at least 1.95 mol %, even more preferably at least 2.0 mol %, most preferably at least 2.1 mol %. If the 1,2-glycol bond content of the polyvinyl alcohol is smaller than 1.9 mol %, the aqueous emulsion containing it will be poorly resistant to water. If so, in addition, the emulsion viscosity will much depends on ambient temperatures and the polymerization stability in producing the emulsion will be poor. Preferably, the 1,2-glycol bond content of the polyvinyl alcohol is at most 4 mol %, more preferably at most 3.5 mol %, most preferably at most 3.2 mol %. The 1,2-glycol bond content of the polyvinyl alcohol can be obtained through NMR spectrometry of the polymer.
The vinyl ester monomer includes, for example, vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl isobutyrate, vinyl pivalate, vinyl versatate, vinyl caproate, vinyl caprylate, vinyl laurate, vinyl palmitate, vinyl stearate, vinyl oleate, vinyl benzoate, etc. Especially preferred is vinyl acetate.
For the dispersant, favorable are those produced by hydrolyzing the polymers of vinyl esters mentioned above, such as vinyl acetate. Not interfering with the object and the effect of the invention, the polymers may contain any other monomer units. Additional monomers employable herein include, for example, xcex1-olefins such as propylene, n-butene, isobutylene, etc.; acrylic acid and its salts; acrylates such as methyl acrylate, ethyl acrylate, n-propyl acrylate, iso-propyl acrylate, n-butyl acrylate, iso-butyl acrylate, t-butyl acrylate, 2-ethylhexyl acrylate, dodecyl acrylate, octadecyl acrylate, etc.; methacrylic acid and its salts; methacrylates such as methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, iso-propyl methacrylate, n-butyl methacrylate, iso-butyl methacrylate, t-butyl methacrylate, 2-ethylhexyl methacrylate, dodecyl methacrylate, octadecyl methacrylate, etc.; acrylamide and its derivatives such as N-methylacrylamide, N-ethylacrylamide, N,N-dimethylacrylamide, diacetonacrylamide, acrylamidopropanesulfonic acid and its salts, acrylamidopropyldimethylamine and its salts and quaternary salts, N-methylolacrylamide and its derivatives, etc.; methacrylamide and its derivatives such as N-methylmethacrylamide, N-ethylmethacrylamide, methacrylamidopropanesulfonic acid and its salts, methacrylamidopropyldimethylamine and its salts and quaternary salts, N-methylolmethacrylamide and its derivatives, etc.; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, iso-propyl vinyl ether, n-butyl vinyl ether, iso-butyl vinyl ether, t-butyl vinyl ether, dodecyl vinyl ether, stearyl vinyl ether, etc.; nitriles such as acrylonitrile, methacrylonitrile, etc.; vinyl halides such as vinyl chloride, vinylidene chloride, vinyl fluoride, vinylidene fluoride, vinyl bromide, etc.; allyl compounds such as allyl acetate, allyl chloride, sodium allylsulfonate, etc.; unsaturated carboxylic acids such as fumaric acid, maleic acid (anhydride), itaconic acid, and their salts and esters, etc.; vinylsilyl compounds such as vinyltrimethoxysilane, etc.; isopropenyl acetate; tetrafluoroethylene; sodium vinylsulfonate; and N-vinyl compounds such as N-vinylpyrrolidone, N-vinylformamide, N-vinylacetamide, etc. Mercapto-terminated or carboxyl-terminated, modified polyvinyl alcohols that are produced by polymerizing a vinyl ester monomer such as vinyl acetate in the presence of a thiol compound such as thiolacetic acid or mercaptopropionic acid followed by hydrolyzing the resulting polymer may also be used in the invention.
The degree of hydrolysis of the polyvinyl alcohol that contains at least 1.9 mol % of 1,2-glycol bonds and serves as the dispersant in the aqueous emulsion of the invention is not specifically defined, but is generally at least 60 mol %, more preferably at least 70 mol %, even more preferably at least 75 mol %. If its degree of hydrolysis is smaller than 60 mol %, the polyvinyl alcohol will lose solubility in water intrinsic to ordinary polyvinyl alcohols. For attaining the object of the invention to obtain good aqueous emulsions, it is desirable that the degree of polymerization (in terms of the viscosity-average degree of polymerization) of the polyvinyl alcohol to be therein falls between 100 and 8000, more preferably between 300 and 3000.
The vinyl ester monomer to constitute the dispersoid in the aqueous emulsion of the invention includes, for example, vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl isobutyrate, vinyl pivalate, vinyl versatate, vinyl caproate, vinyl caprylate, vinyl laurate, vinyl palmitate, vinyl stearate, vinyl oleate, vinyl benzoate, etc. Especially preferred is vinyl acetate.
The polymer (dispersoid) that contains vinyl ester monomer units is a vinyl ester (co)polymer, including polyvinyl esters, and copolymers of vinyl esters with comonomers capable of copolymerizing with vinyl esters. The comonomers capable of copolymerizing with vinyl esters include, for example, olefins such as ethylene, propylene, isobutylene, etc.; vinyl halides such as vinyl chloride, vinyl fluoride, vinylidene chloride, vinylidene fluoride, etc.; acrylic acid, methacrylic acid; acrylates such as methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, dodecyl acrylate, 2-hydroxyethyl acrylate, etc.; methacrylates such as methyl methacrylate, ethyl methacrylate, butyl methacrylate, 2-ethylhexyl methacrylate, dodecyl methacrylate, 2-hydroxyethyl methacrylate, etc.; dimethylaminoethyl acrylate, dimethylaminoethyl methacrylate, and their quaternated derivatives; acrylamide monomers such as acrylamide, methacrylamide, N-methylolacrylamide, N,N-dimethylacrylamide, acrylamido-2-methylpropanesulfonic acid and its sodium and potassium salts, etc.; styrene monomers such as styrene, xcex1-methylstyrene, p-styrenesulfonic acid and its sodium and potassium salts, etc.; other vinyl compounds such as N-vinylpyrrolidone, etc.; and dienic monomers such as butadiene, isoprene, chloroprene, etc. One or more of these comonomers are copolymerized with the vinyl ester monomer, either singly or as combined. Preferably, the amount of the comonomer such as ethylene that is copolymerized with the vinyl ester monomer is at most 50% by weight, more preferably at most 40% by weight of all the monomers to be copolymerized. Of the polymers that contain vinyl ester monomer units, most preferred are polyvinyl esters and vinyl ester-ethylene copolymers.
Regarding the viscosity profile of the aqueous emulsion of the invention, it is important that the ratio of the emulsion viscosity at 0xc2x0 C., T0xc2x0 C., to the emulsion viscosity at 30xc2x0 C., T30xc2x0 C., or that is the ratio of T0xc2x0 C./T30xc2x0 C. is at most 5. The ratio T0xc2x0 C./T30xc2x0 C. is measured according to the method mentioned hereinunder. The ratio T0xc2x0 C./T30xc2x0 C. of at most 5 means that the emulsion viscosity increase at around 0xc2x0 C. is retarded, or that is, the temperature dependence of the emulsion at around 0xc2x0 C. is small. More preferably, T0xc2x0 C./T30xc2x0 C. is at most 4, even more preferably at most 3.
In the invention, it is also important that the ratio of the emulsion viscosity at 60xc2x0 C., T60xc2x0 C., to the emulsion viscosity at 30xc2x0 C., T30xc2x0 C., or that is the ratio of T60xc2x0 C./T30xc2x0 C. is at most 1.5. The ratio T60xc2x0 C./T30xc2x0 C. is measured according to the method mentioned hereinunder. The viscosity of vinyl ester polymer-containing aqueous emulsions often increase at around 60xc2x0 C. In the invention, however, the ratio T60xc2x0 C./T30xc2x0 C. is defined to be at most 1.5. This means that the emulsion viscosity increase at around 60xc2x0 C. is retarded, or that is, the temperature dependence of the emulsion at around 60xc2x0 C. is small. More preferably, T60xc2x0 C./T30xc2x0 C. is at most 1.3, even more preferably at most 1.2. To that effect, the temperature dependence of the aqueous emulsion of the invention is small both at low temperatures and at high temperatures, and the workability and the handlability of the aqueous emulsion are extremely good.
The aqueous emulsion of the invention can be obtained by polymerizing a vinyl ester monomer through emulsion polymerization in the presence of (1) a polyvinyl alcohol that contains at least 1.9 mol % of 1,2-glycol bonds and serves as a dispersant, and (2) at least one polymerization initiator selected from hydrogen peroxide, ammonium persulfate and potassium persulfate in a molar ratio to the vinyl ester monomer of from 0.001 to 0.03, in such a polymerization mode that (3) from 5 to 20% by weight of the vinyl ester monomer, based on the total amount of the monomer (that is, based on the total amount of the vinyl ester monomer to be polymerized), is fed into the reactor in the initial stage of polymerization and the polymerization initiator is fed thereinto all at a time in a molar ratio to the initial feed of the vinyl ester monomer of from 0.001 to 0.05.
To attain the object of the invention, the amount of the polyvinyl alcohol having at least 1.9 mol % of 1,2-glycol bonds to be used in the polymerization process preferably falls between 1 and 20 parts by weight relative to 100 parts by weight of the vinyl ester monomer (in case where the monomer is combined with a comonomer capable of copolymerizing with it, the total of the monomer and the comonomer), more preferably between 3 and 20 parts by weight, even more preferably between 5 and 15 parts by weight. One characteristic feature of the invention is that, even when the amount of the polyvinyl alcohol having at least 1.9 mol % of 1,2-glycol bonds used in the polymerization process is small, for example, even when the amount of the polyvinyl alcohol used therein falls between 1 and 5 parts by weight, it surely attains the object of the invention.
According to the polymerization process, obtained is the aqueous emulsion that satisfies the requirements of T0xc2x0 C./T30xc2x0 C. of at most 5 and T60xc2x0 C./T30xc2x0 C. of at most 1.5.
As is obvious from the Examples given hereinunder, another characteristic feature of the polymerization process employed in the invention is that the filtration residue after emulsion polymerization therein is small, and this means good polymerization stability in the process.
In producing the aqueous emulsion of the invention, it is important to use at least one polymerization initiator selected from hydrogen peroxide, ammonium persulfate and potassium persulfate. Of these, especially preferred is hydrogen peroxide. In the step of emulsion polymerization in the invention, it is also extremely important to define the molar ratio of the polymerization initiator to the vinyl ester monomer to fall between 0.001 and 0.03, preferably between 0.0015 and 0.025, more preferably between 0.0018 and 0.023.
As the case may be, the polymerization initiator will be combined with a reducing agent to give a redox system reagent for use in the invention. In that case, in general, hydrogen peroxide will be combined with tartaric acid, L-ascorbic acid, Rongalit or the like; and ammonium or potassium persulfate will be combined with sodium hydrogensulfite, sodium hydrogencarbonate or the like. The amount of the reducing agent is not specifically defined, but may generally fall between 0.05 and 3 equivalents, preferably between 0.1 and 2 equivalents, more preferably between 0.3 and 1.5 equivalents to the polymerization initiator to be combined therewith.
Regarding the mode of adding the polymerization initiator to the reaction system, favorably employed herein is a method of adding it thereto all at a time in the initial stage of polymerization, or that is, adding it in one shot thereto. Concretely, from 5 to 20% by weight of all the vinyl ester monomer to be polymerized is first fed into a reactor in the initial stage of polymerization, and the polymerization initiator is fed thereinto all at a time in a molar ratio to the initial feed of the vinyl ester monomer of from 0.001 to 0.05, preferably from 0.0012 to 0.045, more preferably from 0.0013 to 0.04.
One shot addition of the predetermined amount of the polymerization initiator to the reaction system in the initial stage of polymerization improves the water resistance of the aqueous emulsion produced and lowers the temperature dependence thereof, enabling stable polymerization of the monomer, and the filtration residue from the polymerized emulsion is thereby reduced.
For initial polymerization, the monomer and the polymerization initiator are added to an aqueous solution of the dispersant, and polymerized at a temperature falling between 50 and 70xc2x0 C., preferably between 55 and 65xc2x0 C., for a period of time falling between 5 and 60 minutes, preferably between 10 and 50 minutes. In the initial stage of polymerization, the monomer is preferably fed into the reactor all at a time.
The initial polymerization shall be terminated when the concentration of the remaining vinyl ester monomer (in terms of % by weight of the polymer produced) has reached at most 10%, preferably at most 5%, more preferably at most 1%. The initial polymerization is followed by final polymerization. Also in the stage of final polymerization, the polymerization initiator may be added to the reaction system all at a time (in a mode of one shot addition), but, as the case may be, it may be added thereto in a continuous or intermittent addition mode. The polymerization temperature in the final polymerization stage is preferably higher by from 5 to 30xc2x0 C. than that in the initial polymerization stage. Concretely, it may fall between 55 and 100xc2x0 C., preferably between 60 and 95xc2x0 C., more preferably between 70 and 90xc2x0 C.
The polymerization pressure may be atmospheric pressure both for initial polymerization and final polymerization, but, as the case may be, increased pressure will be needed. In particular, in case where copolymer emulsions of a vinyl ester with any other comonomer, for example, with ethylene are produced, the monomers must be polymerized under increased pressure.
The aqueous emulsion of the invention obtained in the manner as above has improved water resistance and is therefore favorably used for various applications that require water resistance. In addition, the viscosity of the aqueous emulsion of the invention depends little on ambient temperatures and its viscosity does not increase even when it is stored, transported or used at low or high temperatures. Therefore, the aqueous emulsion has the advantage of good processability and handlability.
The aqueous emulsion of the invention produced according to the method as above may be directly used as it is, but, if desired, it may be combined with any other known emulsions not interfering with the object and the effect of the invention.
The dispersant to be in the aqueous emulsion of the invention is the above-mentioned PVA polymer having at least 1.9 mol % of 1,2-glycol bonds. If desired, however, it may be combined with any known anionic, nonionic or cationic surfactants, and hydroxyethyl cellulose. Also not interfering with the object and the effect of the invention, it may be combined with a polyvinyl alcohol of which the 1,2-glycol bond content is smaller than 1.9 mol %.
As the aqueous emulsion is highly resistant to water and its viscosity depends little on ambient temperatures, the aqueous emulsion of the invention is favorably used in various fields of paper-processing adhesives for paper or pulp products such as paper tubes, paper bags, paper laminates, corrugated cardboard, etc.; wood-processing adhesives for flush panels, wood laminates, butt-welded wood plates, plywood products, secondary worked plywood products (for jointing them), other ordinary wood products, etc.; adhesives for plastics; binders for dip-coated paper, non-woven fabrics, etc.; as well as admixtures, jointing agents, coating compositions, paper processing agents, fiber processing agents, etc.
 less than 2 greater than  Next described is the second aspect of the invention, dispersant for suspension polymerization.
The object of the second aspect of the invention can be attained by providing a dispersant for suspension polymerization, which comprises a PVA polymer (A) having at least 1.9 mol % of 1,2-glycol bonds.
One preferred embodiment of the second aspect of the invention is a dispersant for suspension polymerization of vinylic compounds, which comprises a PVA polymer (A) having at least 1.9 mol % of 1,2-glycol bonds and having a degree of polymerization of from 100 to 4000.
Another preferred embodiment thereof is a dispersant for suspension polymerization of vinylic compounds, which comprises a PVA polymer (A) having at least 1.9 mol % of 1,2-glycol bonds and having a degree of polymerization of from 100 to 4000 and a block character of the residual acetic acid group of from 0.35 to 0.8.
Still another preferred embodiment thereof is a dispersant for suspension polymerization of vinylic compounds, which comprises a PVA polymer (A) having at least 1.9 mol % of 1,2-glycol bonds and having a degree of polymerization of from 100 to 4000, a block character of the residual acetic acid group of from 0.35 to 0.8, and a methanol soluble content of from 0.05 to 40% by weight.
Still another preferred embodiment thereof is a dispersant for suspension polymerization of vinylic compounds, which comprises the PVA polymer (A) as above and a polyvinyl ester polymer (B) having a degree of hydrolysis of smaller than 60 mol %, in a ratio by weight of the component (A) to the component (B), (A)/(B) falling between 40/60 and 95/5.
The PVA polymer (A) having at least 1.9 mol % of 1,2-glycol bonds for use in the invention is not specifically defined for its production, and may be produced in any known method. For example, it may be produced by copolymerizing a vinylene carbonate with a vinyl ester monomer to have the 1,2-glycol bond content as above, followed by hydrolyzing the resulting copolymer; or by polymerizing a vinyl ester monomer at a temperature higher than that for ordinary vinyl ester polymerization, for example, at a temperature falling between 75 and 200xc2x0 C., under pressure, followed by hydrolyzing the resulting polymer. In the latter method, the polymerization temperature preferably falls between 95 and 190xc2x0 C., more preferably between 100 and 180xc2x0 C.
For controlling the block character of the residual acetic acid of the PVA polymer for use in the invention to fall between 0.35 and 0.8, for example, the starting polyvinyl ester may be alcoholyzed or hydrolyzed in the presence of a known alkali or acid catalyst. In the process, the block character of the PVA polymer to be produced can be controlled by specifically selecting the type of the hydrolysis catalyst and the type of the solvent to be used. In general, acid hydrolysis gives a PVA polymer having a higher block character than alkali hydrolysis. The block character of the PVA polymer obtained through alkali hydrolysis may be further increased through heat treatment of the polymer. For the PVA polymer for use herein, most preferred is hydrolysis with a sodium hydroxide (NaOH) catalyst in a solvent of methanol, as it is simple.
For controlling the methanol soluble content of the PVA polymer for use in the invention to fall between 0.1 and 40% by weight, various methods such as those mentioned below may be employed either singly or as combined. For this, concretely mentioned are a method of washing the hydrolyzed PVA polymer with an organic solvent such as acetone, methanol, methyl acetate or the like, in which the type of the solvent to be used, the ratio of the solvent to the PVA polymer, the washing time and the washing temperature are specifically controlled so as to make the thus-washed PVA polymer have a predetermined methanol soluble content; a method of adding a PVA polymer having a low degree of polymerization and/or having a low degree of hydrolysis to the washed and/or non-washed PVA polymer; a method of adding an agent for controlling a degree of polymerization, such as acetaldehyde, butyraldehyde, trichloroethylene, perchloroethylene, mercaptans or the like, to the polymerization system that gives a non-hydrolyzed polyvinyl ester polymer, to thereby make the polymer contain an additional component having a low degree of polymerization; a method of multi-stage polymerization to give a non-hydrolyzed polyvinyl ester polymer, in which the degree of polymerization of the polymer produced in every polymerization stage is specifically controlled; and a method of producing a non-hydrolyzed polyvinyl ester polymer through polymerization, in which the polymerization conversion of the polymer produced is specifically controlled.
The vinyl ester monomers for the PVA polymer (A) for use herein may be the same as those for the dispersant in the aqueous emulsion of the first aspect of the invention mentioned hereinabove, and especially preferred is vinyl acetate.
Not interfering with the object and the effect of the invention, the PVA polymer (A) for use herein may contain any other monomer units such as those mentioned hereinabove for use in the dispersant of the aqueous emulsion of the first aspect of the invention.
For producing the vinyl ester polymers through polymerization, employable is any known mode of solution polymerization, bulk polymerization, suspension polymerization or emulsion polymerization, like for those mentioned hereinabove. Depending on the polymerization mode employed, the polymerization initiator to be used for producing the polymers may be selected from azo initiators, peroxide initiators and redox initiators.
If desired, the PVA polymer (A) for use herein may be modified by introducing thereinto an ionic group such as an ammonium group, a carboxyl group or a sulfone group to thereby increase its solubility in water, or may be modified by introducing thereinto a nonionic group or a (long-chain) alkyl group or the like. Of such modified polymers, the degree of hydrolysis shall be determined from the ratio of the vinyl alcohol group to the vinyl ester group in the polymers, and shall not include the degree of hydrolysis of the ionic group, the nonionic group or the (long-chain) alkyl group introduced into the polymers. Terminal-modified polymers obtained in a known method of polymerizing a vinyl ester monomer such as vinyl acetate in the presence of a thiol compound such as thiolacetic acid or mercaptoproionic acid followed by hydrolyzing the resulting polymer are also employable herein.
The PVA polymer (A) for use in the invention has at least 1.9 mol % of 1,2-glycol bonds in the polymer chain. Preferably, the 1,2-glycol bond content of the PVA polymer (A) is at least 1.95 mol %, more preferably at least 2.0 mol %, most preferably at least 2.1 mol %. If the 1,2-glycol bond content of the PVA polymer (A) in its polymer chain is smaller than 1.9 mol %, the PVA polymer (A) could not ensure good polymerization stability in suspension polymerization of vinylic compounds, when its amount added to the polymerization system is reduced. Therefore, the amount of the PVA polymer (A) to be added thereto must be increased. If so, in addition, the vinyl polymer obtained through suspension polymerization in the presence of the PVA polymer (A) will be readily yellowed, the residual PVA polymer (A) in the polymerization waste water will increase and pollute the environment, and the polymerization system will foam greatly. The 1,2-glycol bond content of the PVA polymer (A) in its polymer chain is preferably at most 4 mol %, more preferably at most 3.5 mol %, most preferably at most 3.2 mol %. If it oversteps 4 mol %, the productivity of the PVA polymer (A) will lower.
The viscosity-average degree of polymerization (hereinafter referred to as xe2x80x9cdegree of polymerizationxe2x80x9d) of the PVA polymer (A) for use in the invention preferably falls between 100 and 4000. More preferably, its lowermost limit is at least 150, even more preferably at least 200. Its uppermost limit is more preferably at most 3500, even more preferably at most 3000. If the degree of polymerization of the PVA polymer (A) is lower than 100, the polymerization stability in suspension polymerization of vinylic compounds in the presence of the polymer will be not good; but if larger than 4000, the PVA polymer (A) will be difficult to handle and its productivity will be low.
The block character of the residual acetic acid group of the PVA polymer (A) for use in the invention preferably falls between 0.35 and 0.8, more preferably between 0.35 and 0.75, even more preferably between 0.4 and 0.7. If its block character is smaller than 0.35, the PVA polymer (A) will be ineffective for stabilizing the suspension polymerization of vinylic compounds; but if larger than 0.8, the plasticizer absorbability of the vinylic polymers produced will lower and the particle size distribution of the vinylic polymers will broaden. If so, in addition, the productivity of the PVA polymer (A) will be low. The block character of the residual acetic acid group of PVA referred to herein is an index that indicates the chain distribution condition of the residual acetic acid group of partially-hydrolyzed PVA, and a method for determining it is described in detail in Poval (published by the Polymer Publishing in 1984) and in Macromolecules, 10, 532 (1977).
The methanol soluble content of the PVA polymer (A) for use in the invention preferably falls between 0.05 and 40% by weight. Its lowermost limit is more preferably at least 0.1% by weight, even more preferably at least 0.15% by weight. Its uppermost limit is more preferably at most 37% by weight, even more preferably at most 35% by weight. If the methanol soluble content thereof is smaller than 0.05% by weight, the productivity of the PVA polymer (A) will be low; but if larger than 40% by weight, the PVA polymer (A) will be readily yellowed and it will therefore yellow the vinylic polymers to be produced in the presence of it.
The degree of hydrolysis of the PVA polymer (A) for use in the invention is not specifically defined. However, in case where the PVA polymer (A) is used herein as the primary dispersant, the degree of hydrolysis thereof is preferably at least 60 mol %. Its lowermost limit is more preferably at least 65 mol %, even more preferably at least 68 mol %. Its uppermost limit is preferably at most 98 mol %, more preferably at most 95 mol %, even more preferably at most 90 mol %. If the degree of hydrolysis of the polymer is smaller than 60 mol %, the solubility thereof in water will be low and the polymer will be difficult to handle. If so, therefore, the capability of the polymer to serve as a protective colloid will be poor. In case where it is used as the primary dispersant, the PVA polymer (A) is preferably soluble in water at 5 to 100xc2x0 C., more preferably at 10 to 90xc2x0 C.
In case where the PVA polymer (A) is used herein as the secondary dispersant for suspension polymeriazation, the degree of hydrolysis thereof is preferably smaller than 60 mol %. Its uppermost limit is more preferably at most 57 mol %, even more preferably at most 55 mol %. Its lowermost limit is preferably at least 20 mol %, more preferably at least 25 mol %. If the degree of hydrolysis thereof is 60 mol % or higher, the polymer will be ineffective for improving the plasticizer absorbability of the vinylic polymers produced in the presence of it. In case where the PVA polymer (A) is used as the secondary dispersant for suspension polymerization, it is desirable that this is combined with the above-mentioned PVA polymer (A) having a degree of hydrolysis of at least 60 mol % and serving as the primary dispersant. Not limited to this, however, the secondary dispersant for suspension polymerization of the PVA polymer (A) may be combined with any other ordinary PVA polymer serving as the primary dispersant.
Also preferably, the dispersant for suspension polymerization of the invention comprises the PVA polymer (A) having at least 1.9 mol % of 1,2-glycol bonds and a polyvinyl ester polymer (B) having a degree of hydrolysis of smaller than 60 mol %, in a ratio by weight of the component (A) to the component (B), (A)/(B) falling between 40/60 and 95/5. The lowermost limit of the ratio of (A)/(B) is more preferably at least 50/50, even more preferably at least 60/40; and the uppermost limit thereof is more preferably at most 90/10, even more preferably at most 80/20. In this, the polyvinyl ester polymer (B) serves as the secondary dispersant for suspension polymerization. Combining the PVA polymer (A) with the polyvinyl ester polymer (B) enables one to obtain vinylic polymers having good plasticizer absorbability.
The polyvinyl ester polymer (B) can be produced in any known method of polymerizing a vinyl ester monomer followed by hydrolyzing the resulting polymer. For this, a vinyl ester monomer may be polymerized under the same condition as that for the PVA polymer (A). For hydrolyzing the resulting polymer into the polyvinyl ester polymer (B), employable is ordinary alcoholysis or hydrolysis with a known alkali or acid catalyst. For this, most preferred is hydrolysis with an NaOH catalyst in a solvent of methanol, as it is simple.
The vinyl ester monomers for the polyvinyl ester polymer (B) may be the same as those for the PVA polymer (A) mentioned above, and especially preferred is vinyl acetate.
Not interfering with the object and the effect of the invention, the polyvinyl ester polymer (B) may contain any other monomer units such as those mentioned hereinabove for the PVA polymer (A).
For producing the vinyl ester polymer (B) through polymerization, employable is any known mode of solution polymerization, bulk polymerization, suspension polymerization or emulsion polymerization, as in the above. Depending on the polymerization mode employed, the polymerization initiator to be used for producing the polymer may be selected from azo initiators, peroxide initiators and redox initiators.
The polyvinyl ester polymer (B) for use herein is insoluble in water or dispersible in water. It may be modified by introducing thereinto an ionic group such as an ammonium group, a carboxyl group or a sulfone group to thereby make the polymer self-emulsifiable, or may be modified by introducing thereinto a nonionic group or a (long-chain) alkyl group or the like. Terminal-modified polymers obtained in a known method of polymerizing a vinyl ester monomer such as vinyl acetate in the presence of a thiol compound such as thiolacetic acid or mercaptoproionic acid followed by hydrolyzing the resulting polymer are also employable herein.
The degree of hydrolysis of the polyvinyl ester polymer (B) for use herein is smaller than 60 mol %. Its uppermost limit is more preferably at most 57 mol %, even more preferably at most 55 mol %; and its lowermost limit is preferably at least 20 mol %, more preferably at least 25 mol %. If the degree of hydrolysis of the polyvinyl ester polymer (B) is 60 mol % or more, the vinylic polymers produced could not have improved plasticizer absorbability.
In case where the PVA polymer (A) is used as the primary dispersant in the invention, the 1,2-glycol bond content of the polyvinyl ester polymer (B) to be combined with it is preferably at least 1.9 mol %, more preferably at least 2.0 mol %, most preferably at least 2.1 mol %. Also preferably, the block character of the residual acetic acid group of the polymer (B) falls between 0.35 and 0.8, more preferably between 0.35 and 0.75, even more preferably between 0.4 and 0.7. Controlling the 1,2-glycol bond content and/or the block character of the residual acetic acid group of the polymer (B) for use herein enables more stable polymerization of vinylic compounds, and more effectively prevents the polymerization system from foaming.
The ratio by weight of the PVA polymer (A) to the polyvinyl ester polymer (B) in this embodiment, (A)/(B) falls between 40/60 and 95/5, preferably between 50/50 and 90/10, more preferably between 60/40 and 80/20. If the ratio by weight of the component (A) to the component (B), (A)/(B) is larger than 95/5, the vinylic polymers produced in the presence of the combination of (A) and (B) could not have improved plasticizer absorbability; but if smaller than 40/60, the combination of (A) and (B) will lose the ability to stabilize the polymerization of vinylic compounds.
The viscosity-average degree of polymerization of the polyvinyl ester polymer (B) for use in the invention is preferably at most 1000. Its lowermost limit is preferably at least 200, more preferably at least 230. Its uppermost limit is more preferably at most 700, even more preferably at most 600. If the degree of polymerization of the polymer (B) oversteps 1000, it is often unfavorable since the solubility of the polymer (B) in water is poor and the dispersibility thereof is also poor.
Next described is the method of producing vinylic polymers through suspension polymerization of vinylic compounds in the presence of the dispersant of the invention. The temperature of the aqueous medium in the method of producing vinylic polymers is not specifically defined. Not only cold water at about 20xc2x0 C. but also hot water at 90xc2x0 C. or higher is favorable to the production method. In case where the polymerization is effected according to a hot-charge process, more preferably used is hot water at 40 to 95xc2x0 C., even more preferably at 50 to 90xc2x0 C. The hot aqueous medium may be not only pure water but also an aqueous solution containing various additives. In addition, it may be an aqueous medium containing any other organic solvent. The amount of the hot aqueous medium to be fed into the polymerization system may be such that the medium fed thereinto can sufficiently heat the polymerization system. For increasing the efficiency of heat removal from the polymerization system, favorable is a method of using a reflux condenser-equipped polymerization reactor, or a hot-charge process of using a reflux condenser-equipped polymerization reactor. The temperature for the suspension polymerization preferably falls between 30 and 80xc2x0 C., more preferably between 40 and 75xc2x0 C.
In the suspension polymerization of vinylic compounds, the amount of the dispersant ((A) or the total of (A) and (B)) to be used is not specifically defined. In general, however, the amount of the dispersant is preferably at most 5 parts by weight, relative to 100 parts by weight of the vinylic compound to be polymerized, more preferably at most 1 parts by weight, even more preferably at most 0.5 parts by weight, still more preferably at most 0.1 parts by weight, further more preferably at most 0.09 parts by weight. The lowermost limit of the amount of the dispersant to be used is preferably at least 0.01 parts by weight, more preferably at least 0.02 parts by weight. In the suspension polymerization of vinylic compounds in the presence of the dispersant of the invention, the ratio of the components to be fed into the reactor and the polymerization temperature may be determined in accordance with the condition generally employed in ordinary suspension polymerization of vinylic compounds such as vinyl chloride.
Using hot water in the suspension polymerization of vinylic compounds is preferred. In addition, it is also preferable to previously heat the vinylic compounds before they are fed into the polymerization reactor.
The dispersant of the invention may be combined with any of ordinary PVA, water-soluble cellulose ethers such as methyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxypropylmethyl cellulose, etc.; water-soluble polymers such as gelatin, etc.; oil-soluble emulsifiers such as sorbitan monolaurate, sorbitan trioleate, glycerin tristearate, ethylene oxide-propylene oxide block copolymers, etc.; water-soluble emulsifiers such as polyoxyethylene sorbitan monolaurate, polyoxyethylene glycerin oleate, sodium laurate, etc. Their amount to be added is not specifically defined, but preferably falls between 0.01 and 1.0 part by weight relative to 100 parts by weight of the vinylic compounds to be polymerized such as vinyl chloride.
If desired, any other additives may also be added to the polymerization system. The additives include, for example, agents for controlling a degree of polymerization such as acetaldehyde, butyraldehyde, trichloroethylene, perchloroethylene, mercaptans, etc.; polymerization inhibitors such as phenolic compounds, sulfur compounds, N-oxide compounds, etc.; and pH-controlling agents, scale inhibitors and crosslinking agents. Any of these additives may be optionally added to the polymerization system. If desired, two or more these additives may be combined. On the other hand, the polymerization initiator for the suspension polymerization may be any ordinary one which is soluble in vinylic compounds and is generally used for polymerization of vinylic compounds such as vinyl chloride. It includes, for example, percarbonate compounds such as diisopropyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, diethoxyethyl peroxydicarbonate, etc.; perester compounds such as t-butyl peroxyneodecanate, xcex1-cumyl peroxyneodecanate, t-butyl peroxyneodecanate, etc.; peroxides such as acetylcyclohexylsulfonyl peroxide, 2,4,4-trimethylpentyl 2-peroxyphenoxyacetate, etc.; azo compounds such as 2,2xe2x80x2-azobisisobutyronitrile, azobis-2,4-dimethylvaleronitrile, azobis(4-methoxy-2,4-dimethylvaleronitrile), etc. These may be combined with any of potassium persulfate, ammonium persulfate and hydrogen peroxide.
The vinylic compounds capable of being polymerized through suspension polymerization in the presence of the dispersant of the invention concretely include vinyl chloride alone, and also monomer mixtures consisting essentially of vinyl chloride (that is, monomer mixtures containing at least 50% by weight of vinyl chloride). The comonomers capable of being copolymerized with vinyl chloride include vinyl esters such as vinyl acetate, vinyl propionate, etc.; (meth)acrylates such as methyl (meth)acrylate, ethyl (meth)acrylate, etc.; olefins such as ethylene, propylene, etc.; and other monomers capable of copolymerizing with vinyl chloride, such as maleic anhydride, acrylonitrile, itaconic acid, styrene, vinylidene chloride, vinyl ether, etc. In addition, the dispersant of the invention is also applicable to homopolymerization or copolymerization of vinylic compounds not containing vinyl chloride, such as those mentioned above.
 less than 1 greater than  First described in detail is the aqueous emulsion of the first aspect of the invention, with reference to the following Examples 1 to 7 and Comparative Examples 1 to 7. In the following Examples and Comparative Examples, xe2x80x9cpartsxe2x80x9d and xe2x80x9c%xe2x80x9d are all by weight unless otherwise specifically indicated. The water resistance (water resistant bonding strength) and the temperature dependence of the viscosity of the emulsions obtained were evaluated according to the methods mentioned below.
(Evaluation of Emulsions)
(1) Water Resistant Bonding Strength (to lumber)
The aqueous emulsion obtained herein is applied onto a piece of straight-grained hemlock fir lumber in an amount of 150 g/m2, and this is stuck on another piece of the same lumber. The two with the emulsion therebetween are pressed under a pressure of 7 kg/m2 for 16 hours. After released from the pressure, this is cured at 20xc2x0 C. and 65% RH for 5 days. Next, this is immersed in cold water at 20xc2x0 C. for 4 days. While still wet, the compression shear strength of the test sample is measured.
(2) Temperature Dependence
T0xc2x0 C./T30xc2x0 C. and T60xc2x0 C./T30xc2x0 C. of each aqueous emulsion are measured, and these indicate the temperature dependence of the emulsion. T0xc2x0 C. is the viscosity of the emulsion measured after conditioned at 0xc2x0 C.; T30xc2x0 C. is the viscosity of the emulsion measured after conditioned at 30xc2x0 C.; and T60xc2x0 C. is the viscosity of the emulsion measured after conditioned at 60xc2x0 C. To measure the viscosity, used is a B-type viscometer (20 rpm).
(3) Polymerization Stability
After having been prepared through polymerization, the aqueous emulsion is filtered through a 60-mesh stainless steel gauze filter, and the filtration residue (%, relative to the emulsion) is measured. A smaller value of the filtration residue thus obtained indicates better polymerization stability in producing the emulsion.